1. Field of the Invention
The present invention relates to a semiconductor device provided with a function to control a current supply to a load by a transistor. In particular, the invention relates to a semiconductor device including a pixel formed of a current driving light emitting element of which luminance changes according to current, and a signal driver circuit thereof.
2. Description of the Related Art
In recent years, what is called a self-light emitting display device of which pixel is formed of a light emitting element such as a light emitting diode (LED) is attracting attentions. As a light emitting element used for such a self-light emitting display device, an organic light emitting diode (OLED), an organic EL element, an electroluminescence (EL) element and the like are attracting attentions and used for an organic EL display and the like.
A light emitting element such as an OLED which is a self-light emitting element is advantageous as compared to a liquid crystal display in that a pixel provides a high visibility, a backlight is not required, a high response speed is achieved, and the like. The luminance of a light emitting element is controlled by a current value supplied thereto.
As a driving method of a display device using such a self-light emitting element, a passive matrix method and an active matrix method are known. With the passive matrix method, a structure is simple but a large display of high luminance is difficult to achieve. The active matrix method in which a current supplied to a light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit is more actively developed recently.
In the case of such an active matrix display device, there is a problem in that a luminance changes when a current supplied to a light emitting element changes due to variations in current characteristics of driving TFTs.
In other words, in the case of such an active matrix display device, driving TFTs which drive a current supplied to a light emitting element is used in a pixel circuit. When the characteristics of these driving TFTs vary, a current supplied to a light emitting element changes, leading to vary the luminance. In view of this, various circuits for suppressing variations in luminance have been suggested, in which a current supplied to a light emitting element does not change even when characteristics of driving TFTs in a pixel circuit vary.